1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus mounted on a motor vehicle and assisting a steering force by an electric motor.
2. Description of the Related Art
A steering torque control of a motor-driven power steering apparatus is basically structured so as to apply an assist force corresponding to a steering torque applied to a steering shaft of a steering mechanism from a motor so as to assist a steering force at a time of steering a steering wheel.
For example, in a motor-driven power steering apparatus disclosed in Japanese Patent Application Laid-open No. 2002-120743 (patent document 1), a target steering torque is estimated based on a steering angle of the steering and a vehicle speed, and a feedback control of a motor is executed based on a deviation between the target steering torque and a detected steering torque.
In the steering torque control disclosed in the patent document 1, a relation of the target steering torque with respect to the steering angle of the steering is previously determined and stored per a vehicle speed, and the target steering torque is extracted and estimated from a memory means based on the steering angle and the vehicle speed.
Since a restoring force is naturally applied to the traveling vehicle based on a self-aligning torque which the traveling wheel receives from a road surface in such a manner as to make the steering angle close to 0 (a neutral position), it is possible to assist the steering force by obtaining a torque overcoming the self-aligning torque by driving the motor, at a time of turning the wheel.
Since the self-aligning torque is different in accordance with the steering angle and the vehicle speed, it is possible to suitably assist the steering force by previously determining the target steering torque with respect to the steering angle per the vehicle speed and driving and controlling the motor based on the target steering torque.
However, since the self-aligning torque is small particularly in the low vehicle speed, a friction of a tire wheel applied to the road surface, a resistance of a power transmission system from the steering wheel and an inertia of the motor greatly act on the steering force, and a steering feeling becomes unstable.